Thank you for choosing Ask an Expert for help with your new houseplant, a monstera, I believe.

Your diagnosis of a bacterial attack seems likely. Bacterial diseases are encouraged by high humidity, wet leaves, and poor air circulation around the plant. You may want to cut off the affected leaves to prevent spreading. A copper spray can also help keep this disease in check.

Another possibility is a fungus disease. In that case, you might see a concentric ring pattern in the brown spots. I do not see this in the photos, but you might see it in person. Again, providing good air circulation and keeping water off the leaves (as with a bacterial disease), and a spray with a copper fungicide (again, as with a bacterial disease) could be in order.

Regards,